1. Field of the Invention
The present invention relates to a cylinder head of an internal combustion engine, and particularly, to a cylinder head in which an additional mechanism, such as a variable valve timing mechanism or the like, is incorporated into a valve system of the engine.
2. Description of the Related Art
In a manufacturing line for an internal combustion engine, a cylinder head as one of parts of the engine is generally manufactured by consecutively processing a workpiece to be the cylinder head, thereby improving productivity of the automobile and standardizing the quality of the cylinder head. As one of such a manufacturing line for the cylinder head, there is a common manufacturing line in which different kinds of prior cylinder heads, e.g., both a single overhead camshaft (SOHC) type cylinder head and a double overhead camshaft (DOHC) type cylinder head, without any additional mechanism, such as a variable timing mechanism can be processed, in order to use one manufacturing line for general purposes.
But, there is a special kind of cylinder head in which a valve system of the engine is arranged with an additional mechanism, such as a variable valve timing mechanism, and this kind of cylinder head has naturally a height greater than the heights of the conventional type cylinder heads of the SOHC and DOHC types in order to allow the inner space thereof for incorporating the variable valve timing mechanism. Therefore, the common manufacturing line cannot be used to process this special kind of cylinder head.
In the common manufacturing line, the maximum size of a processible cylinder head is predetermined. Specifically, the maximum height, width, and length of the processible cylinder head are restricted by the size of a DOHC type cylinder head, since the DOHC type cylinder head typically has a greater height than an SOHC type cylinder head. However, a special cylinder head in which the valve system with the variable valve timing mechanism is arranged, needs to have a height greater than the maximum height defined by the DOHC type cylinder head. As a result, this kind of special cylinder head cannot be processed in the common manufacturing line, even though most parts of this cylinder head, e.g., parts surrounding inlet and exhaust valves are formed so as to comply with the same specification as that of the DOHC type cylinder head.
Thus, in order to process the special cylinder head which cannot be processed in the common manufacturing line, various modifications need to be made to the manufacturing line, or a new special manufacturing line must be provided. However, modifications of the common manufacturing line or provision of a new manufacturing line require not only high costs, but also a long preparation period for starting up the production of the special cylinder head.
It may be possible to manufacture such a special cylinder head by a general purpose machine, without using the special manufacturing line. However, in this case, the productivity is reduced to a low level.